The submarine Seehund or Seal was the most successful mini submarine of the German Kriegsmarine. While the Molch and Biber were rushed into service, the Seehund was build from scratch with all lessons learned and ready for action in late 1944. The U-Boat, named type 127 or XXVII B5, was built by Fisher and Grim and had excellent technical features. The midget submarine was almost 12 meters long, had a beam of 1,7 meters and a diameter of 1,3 meters. The big difference with its predecessors was that it took two operators on board, a pilot seated in front of the navigator.
The Seehund had a maximum range of 555 kilometers with a speed of 13 km/h surfaced and 115 kilometers on a speed of 5,5 km/h submerged. When a external fuel tank was added the maximum range of the Seehund was extended to 925 kilometers. Its save diving depth was 30 meters with a depth record of 70 meters. Like the Biber it carried two torpedo’s of the G7e type on its lower side. It could attack even in though weather with 4 Beufort but hat to lay stationary when attacking submerged. The diesel engine, made by Bussing, delivered 60 hp and the AEG electric motor gave 25 hp. When the diesel engine was converted to a closed – cycle propulsion it produced 100hp and enhanced the speed up to 15 km/h surfaced and almost 20 km/h submerged. The Seehund 227 as this type was designated was faster underwater with this power plant than the 127 type.
The plan was to build 1000 of the Seehund Klein U-Boot as, the Germans called their midget submarine class. Construction of the mini submarines started in 1944 and in September 1944 the first Seehund U-Boats were delivered, mostly constructed by the Schichau and G.W. Howaldt factories but more factories participated in the construction of the Seehund. In total 285 Seehund midget submarines would be built before the end of the war, the last Seehund came from the production line in April 1945.
Because of his small size the Seehund was hard to detect, it did not show up on the Allied radar. It could withstand depth charges, due to its small size it was pushed out of the way and tossed around instead of being destroyed. It could malfunction after a depth charge assault, the sealed off coal dioxide compartment could get open and poisoning the pilot and navigator. Converted it could lay mines as well. In 1945 the Seehund sailed 142 sorties and sinking 8 ships and damaging another 3 ships during this period with a loss of 35 vessels. The most of these 35 Seehund U-boats sank due to technical failure.